1. Field of the Invention
The present invention relates to a brake drum for use in the brakes which are provided in an automatic transmission in a vehicle and the like.
2. Description of the Related Art
In a conventional brake drum, since no groove is formed in a slide surface with respect to a brake band, a proper lubrication condition cannot be provided in a friction surface thereof. Accordingly, it is difficult to stabilize the friction property of the brake band as well as to enhance the heat resistance and durability of the brake band.
In view of this, conventionally, there is proposed a structure in which a spiral-shaped (screw-shaped) groove is formed on the slide surface of the brake drum. An example of such structure is shown in FIG. 2. In FIG. 2, reference character 10 designates a brake band, while 12 stands for a belt-shaped member, and 14 expresses another belt-shaped member which is located inside the belt-shaped member 12. Also, 20 designates a brake drum, and 22 stands for a spiral groove formed on the slide surface of the brake drum 20. In the structure shown in FIG. 2, the spiral groove 22 includes in the right and left portions thereof two grooves 22a and 22b which are different in the spiral directions thereof from each other.
If the spiral-shaped groove is formed on the slide surface of the brake drum in this manner, then a proper wet condition can be formed on the friction surface of the brake band, which in turn makes it possible to stabilize the friction property of the brake band as well as to enhance the heat resistance and durability of the brake band. In FIG. 3, there is shown the torque waveform comparison of the above-mentioned conventional drums, where the horizontal axis expresses a time t and the vertical axis expresses a torque T. In FIG. 3, a dotted line A shows a torque waveform obtained when the conventional brake drum including no groove thereon, whereas a solid line B illustrates a torque waveform obtained when the conventional brake drum including a groove thereon. Also, in this graphical representation, a point a designates a point of starting of brake application, whereas a point b stands for a point of stop of rotation.
In the conventional groove-less brake drum shown by the dotted line A, the biting torque at the brake start point a is high but it tends to decrease as the brake works on, which unfavorably results in the unstabilized friction property. On the other hand, in the conventional grooved brake drum, as shown by the solid line, between the two points a and b, the torque remains unchanged and thus the property thereof can be stabilized.
Further, in FIG. 4, there is shown the relation between the torque and brake band pressure. In FIG. 4, the horizontal axis expresses a time t, and the vertical axis stands for the torque T and brake band pressure F; and, a dotted line shows the torque obtained when the conventional groove-less brake drum is used, and a solid line B shows the torque obtained when the conventional grooved brake drum is used, while a solid line C shows the brake band pressure.
In the groove-less brake drum shown by the dotted line A, as shown by a D portion in FIG. 4, at the point a where braking is started, an initial torque is increased suddenly to thereby cause a speed change shock; whereas, in the grooved brake drum, as shown by the solid line B, the torque varies gently so that the speed change shock can be reduced.
In a torque-time curved line shown in FIG. 5, if it is assumed that a friction coefficient at a point a where braking is started is expressed as .mu..sub.d and a friction coefficient at a point b where rotation is stopped is expressed .mu..sub.o, in order to provide a stable friction property, the nearer to 1 a ratio of .mu..sub.o /.mu..sub.d is, the better.
In FIG. 6, if it is assumed that the horizontal axis expresses a groove depth S and the vertical axis expresses the ratio of .mu..sub.o /.mu..sub.d, at a point A where the groove depth S is 0 (that is, no groove is formed), the ratio of .mu..sub.o /.mu..sub.d is smaller than 1. However, if a value obtained when the brake drum includes a groove is expressed by a point B, then the ratio of .mu..sub.o /.mu..sub.d can be made to approach 1 by selecting a proper value for the groove depth S.
In FIG. 7, if it is assumed that the horizontal axis expresses a groove pitch p and the vertical axis expresses the ratio of .mu..sub.o /.mu..sub.d at a point A where the groove pitch is 0 (that is, no groove is formed), the ratio of .mu..sub.o /.mu..sub.d is smaller than 1. However, if a value obtained when the brake drum includes a groove is expressed by a point B, then the ratio of .mu..sub.o /.mu..sub.d can be made to approach 1 by selecting a proper value for the groove pitch p. As can be seen from FIGS. 6 and 7, the ratio of .mu..sub.o /.mu..sub.d decreases gradually as the groove depth S and groove pitch p increase respectively.
As described above, formation of the spiral-shaped groove on the slide surface of the brake drum can provide a proper lubrication condition on the friction surface of the brake band, thereby being able to stabilize the friction property of the brake band as well as enhance the heat resistance and durability of the brake band. However, because the spiral-shaped groove has a certain angle with respect to the sliding direction of the brake drum due to its screw shape, that is, because the spiral-shaped groove is not parallel to the sliding direction of the brake drum, when such brake drum is used continuously, the friction member or brake band is shaved and is thus worn, which impairs the function of the brake band greatly.